SUPER SPORT 15 Yamaha 70

Optimizing the performance of Boston Whaler boats
dtmackey
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SUPER SPORT 15 Yamaha 70

Postby dtmackey » Sat May 19, 2018 11:11 pm

A SUPER SPORT 15 has a Yamaha 70-HP two-stroke-power-cycle engine. I am looking for suggestions for a propeller. The boat is in project stage now, so I can't test anything, but the engine came with a Yamaha stainless propeller that's painted black and size 13 x 17. The proeller does have dings.

Is a Yamaha stainless 13 x 17 a good size and pitch for a SUPER SPORT 15 with Yamaha 70?

Or, rather than getting the propeller reconditioned, should I apply the cost of reconditioning toward another propeller of another size?

--D
Last edited by dtmackey on Sun May 20, 2018 10:46 pm, edited 1 time in total.

jimh
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Re: SUPER SPORT 15 Yamaha 70

Postby jimh » Sun May 20, 2018 8:48 am

To assess if the cost of reconditioning an existing propeller might be better spent on a new propeller, the two costs would have to be known. If the cost to recondition is more than one-third of the cost of a new propeller, I would begin to favor the new propeller. Generally a reconditioned propeller could mean almost anything. For a very high-quality reconditioning where the propeller was carefully balanced, I suspect the cost would begin to approach a new propeller cost.


The Yamaha steel propellers are of a good all-round design and tend to work well. Avoiding using any propeller with bent or misshaped blades. The unbalance of the propeller blades will cause vibration and tend to work on the propeller shaft seals and bearings. Repair of those engine parts will be more expensive than a new propeller.

dtmackey
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Re: SUPER SPORT 15 Yamaha 70

Postby dtmackey » Sun May 20, 2018 10:44 pm

Thank you for your opinion Jim. The place I bring props for reconditioning has been in business for 50 years and I've walked through their operation on more than one occasion and it's pretty impressive. They do all the local Coast Guard and Gloucester fishing fleet and also have vans that run from NY to Canada to service the many marinas for pleasure boater props. Several years ago we had the prop reconditioned on our larger boat and they did a great job and certified it to Class S, which was better than new, so I have no question of their work. You do bring up a good point and not every prop is repairable and there are some less credible shops that may take on that type of work when business is slow.

The prop I have show minor damage to the leading edge, a very easy fix, but a stainless prop with deep damage, I would recycle and buy new.

Now back to the original post question, anyone have input on the diameter and pitch for a 70hp 2 stroke Yamaha on a 15' Whaler Super Sport?

D-

jimh
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Re: SUPER SPORT 15 Yamaha 70

Postby jimh » Mon May 21, 2018 8:21 am

Has your propeller shop given you an estimate to recondition the Yamaha propeller?

If their estimate to recondition is a reasonable cost, I'd get the Yamaha propeller fixed. My recommendation is based on my experience with those propellers.

I had a pair of Yamaha 70-HP engines on my 20-foot Boston Whaler. They had the Yamaha steel black-paint 17-pitch propellers. The propeller were very good, got the boat on plane easily, never ventilated, and I never gave a thought to fitting something else.

Your description of the propeller shop and its work product is a further encouragement to my recommendation to refurbish the Yamaha propeller--again, if the cost is appropriately in the guidelines I suggested.

I hope my advice in response to your initial question on whether to have the propeller reconditioned or not is useful.

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Re: SUPER SPORT 15 Yamaha 70

Postby jimh » Mon May 21, 2018 8:34 am

dtmackey wrote:...anyone have input on the diameter and pitch for a 70hp 2 stroke Yamaha on a 15' Whaler Super Sport?


Do you have the gear ratio for that engine? Do you have the maximum engine speed range information?

I would estimate a pitch based on the speed potential for the boat with 70-HP. According to the Boston Whaler literature, 70-HP on a 15-footer should produce a top boat speed of around 42-MPH. This will give a target speed, and with a propeller calculator, the engine gear ratio, and the engine maximum RPM, we can estimate a propeller pitch.

For example, if the gear ratio is 2.33:1 (just a guess), if the recommended speed range at full throttle is around 5,500-RPM (another guess), and if the SLIP factor will be 7 (a reasonable value based on experience), then we put the following into the propeller calculator

RPM = 5500
RATIO = 2.33
SLIP = 7
MPH = 42

and let the calculator determine a pitch. The answer is 20-pitch. That sounds a bit tall, so perhaps 19-pitch will be more suitable.

Since I wasn't sure about the gear ratio, I re-ran with RATIO = 2; the outcome of that is 17-pitch.

dtmackey
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Re: SUPER SPORT 15 Yamaha 70

Postby dtmackey » Thu May 24, 2018 11:04 pm

Thanks Jim, I have the prop calculator app on my phone and used it many times on other boats - Great tool. I'm going to go with the 19 pitch based on a conversation with someone else with a similar setup since using the calculator before testing has assumptions build in. I like the motor to run at the top end of the RPM range, if not 100 - 200RPM over so when loaded it still has the umph to get on a plane.

Rigged the motor tonight and it looks like 1 hole up is ideal (top mounting bolt 2nd hole). This put the cavitation plate (ant-ventilation plate for those who prefer) right where I want it so it's flush with where the water should be coming off the bottom of the hull. Ideally you want to see the top of the cavitation plate on plane.

D-

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Phil T
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Re: SUPER SPORT 15 Yamaha 70

Postby Phil T » Fri May 25, 2018 7:10 am

In reviewing the archives, the 13.25 x 17 Yamaha Painted Stainless Steel prop is a good start for your Yamaha 70.

You will want the motor 2 holes up. See http://continuouswave.com/forum/viewtopic.php?f=7&t=739
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jimh
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Re: SUPER SPORT 15 Yamaha 70

Postby jimh » Fri May 25, 2018 3:56 pm

I found a manual on-line for the 70-HP two-stroke-power-cycle Yamaha:

http://www.yamaha-motor.com/assets/serv ... 5_1390.pdf

It shows the gear ratio is 2.33:1 as I guessed above. And WOT engine speed is 5000 to 6000-RPM. With that established, then a 19-pitch should get the boat to the suggested 42-MPH, but you will probably need to hold down the total weight. Try that Yamaha 17-pitch to start before buying another propeller at 19-pitch. The 17-pitch should get the boat speed to about 39-MPH if you can wind up to 6,000-RPM. Depending on the water conditions you plan for boating, 39-MPH might be plenty fast in a 15-footer.

dtmackey
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Re: SUPER SPORT 15 Yamaha 70

Postby dtmackey » Sun May 27, 2018 8:36 pm

Thanks for all the input.

I found a great price on a Turning Point 13.25X19 stainless prop ($200 new in box), so I grabbed it and will give it a shot. If if works out, then I won't have the Yami 17" prop rebuilt. As suggested I'll rig the motor 2 holes up and re-drill for the blind hole in the mounting bracket unless anyone has input if the lower mounting holes have enough exposure to through bolt though the motor well.

D-

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Re: SUPER SPORT 15 Yamaha 70

Postby jimh » Sun May 27, 2018 9:29 pm

For advice on mounting hole location on the transom specific to Boston Whaler older boats with shallow engine splash wells like the classic 15-footer see

http://continuouswave.com/whaler/reference/FAQ/#Q8

If the blind hole is used, the disadvantage is the engine mounting height cannot be changed without drilling a new hole.